The monitoring of analytes in gases and liquids have numerous applications in industrial chemical processes, bioprocessing, fermentation processes, and environmental monitoring of the atmosphere, oceans, lakes, streams and groundwater. Typically, such processes are fairly complex and comprise a number of chemical or biochemical species of interest which are either indicative of performance, beneficial to the process or potentially harmful to the viability of such processes.
It is well known, for example, that bioprocess or fermentation process performance may be evaluated by the production or disappearance of key analytes and measurement of pH, dissolved oxygen, carbon dioxide and glucose. Industrial process performance may be assessed by monitoring of oxygen, carbon dioxide, nitrogen oxides, sulfur oxides, cations such as alkali metals or metals, and anions, such as halides or anion salts. Both indoor and outdoor monitoring of oxygen, carbon dioxide, pH, nitrogen oxides, sulfur oxides, halogens, organic toxins, trace metals and heavy metals are frequent required in assessments of health and safety as well as environmental quality.
Such complex processes typically require continuous monitoring of analytes due to their dynamic nature. Conventional laboratory methods for continuous analysis of process analytes are typically cumbersome and costly, requiring in-situ sampling and off-site analysis which comprise complex separation of analytes from their sample matrix medium. Such sampling methods lack continuity for interfacing with such dynamic processes and preclude immediate feedback of analyte information for real-time process control. Thus, there is a need for cost-effective, real-time, in-situ, dynamic monitoring of such processes over extended time periods of operation.
Over the past decade, fiber optic chemical sensors have extended analytical chemistry capabilities for low cost, real-time, in-situ analysis of analytes in industrial, biological and environmental processes by eliminating the need for intermittent sampling and off-line analysis. Such sensors typically provide for analytes to be detected in their native sample medium without cumbersome separations and tedious sample preparation. These sensors operate by detecting optical changes of a sensing material or indicator dye on interaction with an analyte. Due to the variety of analyte-specific indicators available, such sensors may be used for monitoring a large number of analytes. Arrays of such sensors may be employed with either selective and semi-selective indicators for monitoring a large number of target analytes simultaneously. Due to the small size of the optical fibers employed in such sensors, typically ranging from sub-micron to 500 um in diameter, these sensors may be easily and unobtrusively accommodated in virtually any process or environmental sensing application.
The accurate monitoring of low level pCO.sub.2 (0 to 1000 ppm) is important in many systems. For example, CO.sub.2 is used as an aerial fertilizer in greenhouses; CO.sub.2 enrichment from ambient levels (345 ppm) to 1000 ppm can improve tomato yield by 35% [Hand, D. Grower. 1985, 104 (3), 31]. Similarly, the production or disappearance of CO.sub.2 is a key parameter in assessing the performance of various fermentation and bioreactor processes m the biotechnology industries. Therefore, robust sensing technology for the fast and accurate determination of low level CO.sub.2 is highly desirable.
The measurement of low level pCO.sub.2 is particularly important for environmental monitoring. Both oceanographic and fresh water measurements are important to understand global changes in the environment brought about by the burning of fossil fuels and the destruction of rain forests [Sarmiento, J. L. C&E News. 1993, 30]. Wide-reaching, long term monitoring of pCO.sub.2 is a critical requirement for realistic and predictive modeling of ocean-atmosphere coupling and the balancing of the global CO.sub.2 budget [Siindquist, E. T., Science, 1993, 259, 934]. At present, oceanographic pCO.sub.2 seawater measurements are obtained by research ships using water sampling techniques. Such an approach is expensive and provides low spatiotemporal resolution due to the limited numbers of samples, which can be taken. Thus, there is an immediate need for inexpensive, low-level, high spatiotemporal resolution pCO.sub.2 sensors which can be remotely deployed over large areas for continuous, long-term environmental monitoring.
With conventional methods, CO.sub.2 in the gas phase is usually determined using IR measurements. However, dissolved CO.sub.2 is typically measured by either electrochemical or colorimetric methods, techniques which are not suitable for continuous, long-term, remote, environmental monitoring. Particularly useful alternative methods for environmental monitoring of dissolved CO.sub.2 utilize chemical sensors.
Most chemical sensors for dissolved CO.sub.2, including the innovative sensor described herein, are based on the principles behind the Severinghaus electrode [Severinghaus, J. W., Bradley, A. F., J2 Appi. Physiol. 1956, 13, 515]. This electrochemical sensor consists of a pH electrode in contact with a bicarbonate buffer solution which is confined at the electrode surface by a gas permeable membrane, such as PTFE or silicone rubber. Certain features of the Severinghaus CO.sub.2 electrode design have been incorporated in optical CO.sub.2 sensor designs. With these optical sensor embodiments, the Severinghaus pH electrode is replaced with an absorbance or fluorescence-based pH sensitive indicator coupled to an optical fiber. With either optical or electrochemical sensor designs, the sensor measures the pH of the HCO.sub.3 solution which is in equilibrium with CO.sub.2 outside the membrane according to the following mechanism: ##STR1##
The external CO.sub.2 concentration is related to the internal H.sup.+ concentration by the following equation [Jensen, M. A., Rechnitz, G. A. Anal. Chem. 1979, SI, 515]: EQU h.sup.3 +nh.sup.2 -(K.sub.1 a.sub.T +K.sub.W)h-2K.sub.1 K.sub.2 a.sub.T =0 [1]
where n is the concentration of sodium ions in the internal solution, h=[H.sup.+ ], K.sub.W =h[OH.sup.- ], a.sub.T is the total analytical concentration of carbon dioxide in the indicator solution layer, K.sub.1 =KK.sub.a =hb/a and K.sub.2 =hc/b where b=[HCO.sub.3.sup.- ] and c=[CO.sub.3.sup.2- ].
Fiber optic CO.sub.2 sensors are known in the art for high-level dissolved CO.sub.2 measurement. Both absorbance-based sensor designs [Vurek, G. G., Peterson, J. I., Goldstein, S. R. Severinghaus, J. W., Fed. Proc. Am. Soc. Exp. Biol. 1982, 41, 1484; Mills, A., Chang, Q., McMurray, N. Anal. Chem. 64, 64, 1383] and fluorescence.about.based sensor designs [Munkholm, C. and Walt, D. R., Talanta, 1988, 35, 109; Uttamlal, M., Walt, D. R., Bio/Technology, 1995, 13, 597; Mills, A., Chang, Q. Analyst, 1993, 118, 839; Zhujun, Z., Seitz., W. R. Anal. Chim. Acia. 1984, 160, 305] have been disclosed. However, most of these sensors are suitable only for high CO.sub.2 levels (0.01-1 atm.) and only a few report ppm range sensitivity. While modifications which improve the sensitivity of these conventional fiber optic sensor designs have been disclosed, for example by employing inner filter effects to enhance sensitivity [Walt, D. R, Gabor, G., Goyet, C. Anal. Chim. Actca 1993, 274, 47], such modifications have been limited to improvements in maximum resolution of .+-.7 ppm.
In an alternative approach to modification of conventional fiber optic CO.sub.2 sensors for improved sensitivity, DeGrandpre [DeGrandpre, M. D., Anal. Chem. 1993, 65 (4), 331] has disclosed a sensor that, unlike conventional fixed reagent fiber optic sensors, operates by the constant replacement of the sensing solution at the distal end of the fiber by employing a fluid pumping system. This sensor has improved sensitivity over conventional designs, operating in the 0-1000 ppm CO.sub.2 range with an accuracy of .+-.0.8 ppm. While this sensor design is suitable for low-level pCO.sub.2 measurents, it requires a somewhat cumbersome pumping system for replenishment of sensing solution which adds complexity, required maintenance, and increased sensor costs.
Thus, there is a pressing need for a simple, low cost, low maintenance, reliable and sensitive chemical sensor and sensing method for remote sensing of low-level, dissolved analytes for applications involving the environmental, industrial, chemical, biochemical, and biological monitoring of fluids.